1. Field of the Invention
The present invention relates to a double-ended tuning fork (DETF) and, more particularly, to DETFs used for force transducers whose dimensions are selected to avoid spurious modes of operation in order to increase the mechanical Q of the DETF.
2. Description of the Prior Art
Double-ended tuning forks (DETF) are known to be used in various transducers to measure force, acceleration, pressure and the like. Examples of such DETFs are disclosed in U.S. Pat. No. 4,912,990, hereby incorporated by reference. The DETFs used in such transducers are known to be made from a relatively rigid material, such as crystalline quartz, which provides a relatively high mechanical Q and relatively high sensitivity to axial force along the sensitive axis SA. Such DETFs are formed with a pair of spaced-apart, vibrating beams or tines, joined together at opposing ends. Transducers which utilize such DETFs normally include a pendulum or proof mass, connected to a casing by one or more flexures which enable the pendulum to rotate about a hinge axis HA, defined by the flexures. The DETF is normally connected between the casing and the pendulum in a direction generally perpendicular to the hinge axes HA to define a sensitive axis SA.
Excitation is normally applied to the DETF to cause the vibrating beams to vibrate at a resonant frequency when the pendulum is at rest. Forces applied along the sensitive axis SA cause the vibrating beams to go into either tension or compression, which changes the resonant frequency of vibration of the beams. This change in frequency, in turn, is used to measure the force applied along the sensitive axis SA.
A problem has been discovered relative to DETFs, especially those used in force transducers. In particular, such DETFs have been known to operate in various spurious modes which may develop over the operational range of the transducer. These spurious modes are known to lower the mechanical Q of the transducer, as well as cause a shift in frequency, and even stop the oscillation of the device at the desired natural resonant frequency.
Such spurious modes of operation are known to be associated with certain dimensional ratios of the DETF. In particular, the DETF is formed with a pair of spaced-apart, generally parallel vibrating beams, wherein each beam has a thickness t, a width w, a length L between fixed ends and a beam length m. As disclosed in U.S. Pat. No. 4,372,173, hereby incorporated by reference, certain values of the ratios t/w and L/m can result in spurious modes of operation. In order to avoid such spurious modes of operating, the '173 patent discloses a spurious mode map which graphically illustrates ranges of values for the ratios t/w and L/m in which certain spurious modes of operation can be avoided. Unfortunately, the spurious mode map disclosed in the '173 patent is based upon simple beam theory and is, thus, relatively inaccurate. In addition, the spurious mode map disclosed in the '173 patent does not account for symmetric outriggers in the base region of the DETF, which can increase the mechanical Q of the transducer.